In communication systems employing information coding, such as forward error correction coding, the number of code bits are increased by adding redundancy to information bits, such as by convolution. Redundancy coding, as known in the art, provides more signal elements than necessary to represent the intrinsic information. Channel coding, a type of redundancy coding, is often employed at the expense of using more channel capacity than might otherwise be necessary, to permit improved information recovery over channels that exhibit impairments, such as mobile radio communication channels. Channel coding is well understood in digital communication theory and is used in digital speech radio communication systems, such as digital radio telephone cellular systems. An information signal, such as digital data or a low bit rate encoded speech signal, is processed into a coded digital signal by some predetermined algorithm, and is hereafter referred to as the processed signal.
In a digital cellular system, a switch site must typically communicate multiple digital voice channels or digital data channels to a cell site over landline interconnections. The cell site in turn transmits these voice channels to mobile subscriber units via radio frequency (RF) channels. In an attempt to improve system performance over such heavily impaired mobile RF channels, the information signals typically undergo channel coding at either the cell site or the switch site (or someplace in between) before they are communicated over the RF channel.
In such digital communication systems, the landline communication paths are expensive to install and maintain; consequently, efficient use of these paths is of utmost importance. However, the performance of the system must also be a consideration so that minimum degradation occurs. The location of where the speech compression and the channel coding are accomplished is an important consideration.
For example, FIG. 1 shows a known method of channel coding speech blocks wherein the switch site (100), comprised of a mobile switching center (MSC) (101) communicates one non-processed 64 kilobit per second (kbps) digitized voice channel over a single 64 kbps channel of a 2.048 Mbps megastream interconnect to a cell site (105). At the cell site, the information on the voice channel is digitally compressed into low bit rate speech information bits by a speech transcoder (110), whose average output data rate is 13 kbps. Some of these speech information bits are then sequentially channel coded by the channel coder (115) resulting in a processed signal of 22.8 kbps per voice that is sent to an RF transmitter (120). This method is not cost effective, since each voice channel requires its own 64 kbps path between the switching center and the cell site.
If, however, all the processing was completed at the switch site, a maximum number of two (22.8 kbps+22.8 kbps=45.6 kbps) integral processed voice channels could be multiplexed onto a single 64 kbps channel of the megastream interconnect. The improvement realized is a reduction by two in the amount of landline capacity required to carry the same number of voices. Excessive landline interconnect costs still exist since only two voices are carried per megastream subchannel.
FIG. 2 depicts a known method for providing four voice channels over one 64 kbps landline megastream subchannel by moving the speech transcoder (110) to the switch site (100) and performing only the speech coding (13 kbps per channel) on all four channels before transmitting over the landline path whereafter the channel coding (processing) is provided at the cell site. Consequently, a 52 kbps (4*13 kbps=52 kbps) speech coded data stream is communicated over the landline connection to the cell site (105) where each voice channel then undergoes channel coding via the channel coder (115) resulting in four 22.8 kbps processed signals. These digital signals are then communicated to the transmitter (120) and transmitted over RF channels.
Unfortunately, this can produce a downlink bulk audio delay since a majority of each speech block (each speech block being 260 bits and representing 20 msec of speech) must be transferred to the cell site before the necessary processing may begin. This is because the channel coded bits generated may each be a function of many of the input information bits. The delay to transfer a block of information is about 17 msec (1040 or (260*4) bits of speech at 64 kbps) and is long enough in duration to be undesirable. Accordingly, there exists a need to maximize processed data throughput over a limited capacity communication path while minimizing delays.